1. Field of the Invention
This invention relates to a method and apparatus for determining the flow of drilling fluid from a subaqueous well and, more particularly, relates to a method and apparatus for determining an abnormal drilling condition such as the initiation of a blowout or the occurrence of lost circulation during heaving or vertical movement of a floating vessel from which the drilling operation is being conducted.
2. Description of the Prior Art
In drilling a well, particularly an oil or gas well, using rotary drilling methods, a hollow drill string extends from the surface to the bottom of the well. A drill bit is attached to the lower end of the drill string. Drilling fluid (mud) is circulated from the surface, through the drill string and orifices in the bit to an annulus defined between the drill string and the inner surface of the well. The mud then circulates upward through the annulus to the surface where it enters one or more tanks for processing (e.g., drill cuttings removed, chemicals added) prior to recirculation into the well.
In a drilling operation, the mud has several functions, the most important being to restrain high pressure fluids within various earth formations. Occasionally, the high pressure fluid intrudes into the well and displaces the mud. This initial intrusion is referred to as a kick. If this occurs, it is important that the pressure condition be balanced as soon as possible; otherwise, the high pressure fluid might flow up the well. This condition is known as a blowout. However, if during the drilling operation a weak earth formation is encountered, the hydrostatic pressure of the mud may fracture the rock and the mud may disperse freely into the formation from the well. This is termed lost circulation.
A blowout is most effectively prevented when the kick or initial intrusion of formation fluid is quickly detected and limited before this fluid displaces a significant amount of mud from the well. Similarly, lost circulation is most effectively limited when the initiation of the loss can be quickly detected and counteracted before a significant amount of the mud has flowed from the well into the formation. Time is of the essence in detecting these abnormal drilling conditions which may become dangerous situations.
Two basic methods are commonly used in the drilling industry to detect kicks or lost circulation. One method is based on a determination of the flow of mud from the well. The second method is based on a determination of the volume of mud displaced from or to the well.
The first method is to determine the rate of flow of drilling mud returning from the well and to compare this rate with either (i) the rate of return mud flow at earlier times or (ii) the rate of mud circulating into the well. The former approach is commonly used and is useful since the rate of circulating mud into the well often remains essentially constant for long periods of time. The latter approach has the advantage of compensating automatically for normal changes in mud circulating rate. An increase in the return flow rate of mud from the well above an equivalent increase in the rate of circulation into the well is an indication of a kick. Similarly, an unexplained decrease in the return flow rate is an indication of lost circulation.
The second basic method centers on the determination of the volume of drilling mud contained in mud tanks at the surface which are in fluid communication with the well. These tanks generally fall into one of two categories--active tanks or trip tanks.
Active tanks are those through which mud is circulated for removal of drilled solids and other treatment prior to recirculation. The volume in the active tanks is responsive to differences between the volume of mud pumped into the well and the volume returning from the well. Although a number of normal processes may affect this volume (removal of drilled solids, addition of water or other materials), an unexplained increase in the volume is an indication of a kick while an unexplained decrease is an indication of lost circulation.
The trip tanks are usually much smaller than the active tanks and, therefore, much more sensitive to changes in mud volume. They are connected to the well during periods when no mud is being circulated into the well through the drill string. Such non-circulating periods include (i) times when a kick is suspected and the mud circulation is stopped to conclusively determine if the well is flowing and (ii) times when the drill string is being removed from or returned to the well. This latter operation is known as a trip, hence, the name "trip tank". During trips, the change in volume of the mud in the trip tank is compared with the displacement expected due to insertion or removal of a given length of drill string into or from the well. In this manner, unexplained increases or decreases in trip tank volume may be interpreted as kicks or lost circulation, respectively.
Unfortunately, drilling offshore wells from a floating vessel complicates the monitoring of the return mud rate or surface mud volume. The drilling vessel is connected to the well by a marine riser which serves as an extension of the well between the sea floor and the vessel. The mud returns to the vessel from the well through an annulus defined between the outer surface of the drill string and the inner surface of the marine riser. In order to accommodate the heaving or vertical motion of the vessel, the marine riser usually includes a telescoping section or slip joint.
At sea, the heaving motion of the vessel oscillates the telescoping section thereby extending and contracting it. In this manner, the lower section (below the telescoping section) of the marine riser remains stationary with respect to the sea floor while the upper section of the marine riser oscillates with the vessel. The oscillating motion of the telescoping section increases and decreases the volume of the annulus and, hence, the volume of the mud in the annulus returning from the well. The resulting variations in the annular volume of the telescoping section affect the measurements of the flow from the well if one wishes to monitor the flow above the telescoping section. Typically, this is the case since it is currently impractical to measure the flow in the marine riser below the telescoping section due to the difficulties associated with a rotating drill string.
The maximum and minimum flow rate of the mud induced by the extension and contraction of the marine riser may be several times larger or smaller than the actual or true flow rate from the well. For example, variations may occur in the measured return flow rate of mud from 2000 gallons per minute (gpm) in the reverse direction (when the telescoping section is expanding) to about 5000 gpm in the normal direction (when the telescoping section is contracting) compared to a true return flow rate of mud from the well of about 1500 gpm. In addition, the variations in the volume of mud in the telescoping section induces variations in the volume of mud contained in those tanks in fluid communication with the riser. These variations complicate an accurate assessment of increases or decreases in the tank volume. Therefore, the cyclic variations in the volume of the marine riser caused by the movement of the vessel complicates an accurate assessment of an abnormal drilling condition. The rapid determination of a blowout or lost circulation condition is very difficult without a means to correct for the effects of the variation in the length of the telescoping section if one wishes to monitor the return mud flow or volume above the telescoping section.
Gorsuch, in his U.S. Pat. No. 3,602,322, discloses a system for sensing a variation between the input and output flows of a well above some defined tolerable range. Gorsuch's system is one of the more elementary patents in the field for determining a blowout or lost circulation. However, its application is limited to a motionless system, e.g. onshore. The Gorsuch system cannot effectively deal with variations in the return flow rate of drilling fluid resulting from the heaving motion of the vessel.
The following U.S. Patents have recognized the problem of accurately assessing the true flow rate of the returning drilling fluid when monitoring the flow rate from above the telescoping joint due to the heaving motion of the offshore vessel:
U.S. Pat. No. 3,760,891--Gadbois
U.S. Pat. No. 3,910,110--Jefferies et al
U.S. Pat. No. 3,976,148--Maus et al
The Gadbois system monitors the return flow rate of the mud at the vessel and generates an electrical signal porportional to that return rate. The signal is then monitored and accummulated over preselected, overlapping time intervals and compared with threshold values to determine the occurrence of a kick or lost circulation. The Gadbois system requires the preselection of a time interval over which the accummulation is performed. The time interval is constant. The Gadbois system, however, does not provide for the monitoring of a telescoping section over time periods such that the effect on the final determination of the flow of mud from the well due to the expansion and contraction of the telescoping section is eliminated.
Jefferies et al (U.S. Pat. No. 3,910,110) discloses a system for detecting a kick or lost circulation in a subaqueous well in which the return rate of the mud flowing back to the vessel from the well is measured and an electrical signal is generated proportional to that rate of flow. The electrical signal is modified to compenste for rates of change in the mud volume within the telescoping section. The modified electrical signal is then compared with a second electrical signal proportional to the rate of flow of the mud into the well. U.S. Pat. No. 3,910,110 discloses a system for continuously modifying the electrical signal which compensates for a change in the volume of the flow path caused by the heaving motion of the vessel.
Maus et al (U.S. Pat. No. 3,976,148) also discloses a method and apparatus for determining on board a heaving vessel the flow rate of drilling mud flowing from a well. However in the Maus disclosure, a first, second and third electrical signal are generated which correspond, respectively, to (i) a flow rate of mud flowing through a conduit downstream the telescoping section, (ii) a rate of change in the volume of mud contained within the riser above the point at which the conduit between the mud processing system and riser intersects the riser, and (iii) a rate of change in the volume of the mud in the telescoping section. The first, second and third signals are then correlated to produce a fourth electrical signal proportional to the flow rate of the mud flowing out of the well into the marine riser. U.S. Pat. No. 3,976,148, however, requires the continuous monitoring of the extension and contraction of the telescoping section to accurately assess the rate of change in the volume of the drilling fluid passing through the marine riser.
Other background references of a general interest relating to heave compensation systems and pressure control of drilling fluid returns are;
U.S. Pat. No. 3,809,170--Ilfrey et al.
U.S. Pat. No. 3,811,322--Swenson
U.S. Pat. No. 3,815,673--Bruce et al.
U.S. Pat. No. 3,946,559--Stevenson
U.S. Pat. No. 4,085,509--Bell et al.
U.S. Pat. No. 4,099,536--Dower
U.S. Pat. No. 4,099,582--Bell
U.S. Pat. No. 4,121,806--Iato et al.
U.S. Pat. No. 4,138,886--Lutz et al.